Chapter 8: Heredity
Reproductive processes create new individuals that are similar but subtly different. While asexual reproduction produces some variation, sexual reproduction maximizes the number of successful variations. In this chapter, we'll explore how these variations are created and inherited.
Accumulation of Variation During Reproduction
Inheritance from previous generations provides both a common basic body design and subtle changes for the next generation. When the new generation reproduces, the second generation will have:
- Differences inherited from the first generation
- Newly created differences
Imp Points:
- Asexual reproduction: Very minor differences due to small inaccuracies in DNA copying (like bacteria dividing)
- Sexual reproduction: Greater diversity generated due to combination of genetic material from two parents
Survival and Natural Selection
Do all variations have equal chances of surviving? Obviously not! Different variations provide different advantages:
Question 1
If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Question 2
How does the creation of variations in a species promote survival?
- Environmental adaptation: Different variations help individuals adapt to changing environmental conditions
- Natural selection: Beneficial variations increase survival chances in specific environments
- Species survival: If the environment changes drastically, at least some individuals with suitable variations will survive
- Example: In a heat wave, bacteria with heat-resistant variations will survive better than those without
Heredity
The most obvious outcome of reproduction is still the generation of individuals with similar design. The rules of heredity determine how traits and characteristics are reliably inherited.
Inherited Traits
A child bears all the basic features of a human being but doesn't look exactly like its parents. Human populations show great variation in traits like:
- Eye color
- Hair texture
- Height
- Skin tone
- Earlobe type (free or attached)
Rules for the Inheritance of Traits – Mendel's Contributions
Gregor Johann Mendel (1822–1884)
Mendel was educated in a monastery and studied science and mathematics at the University of Vienna. Despite failing his teaching certificate examination, he continued his scientific quest. He returned to his monastery and started growing peas.
Imp Contribution: Mendel was the first to keep count of individuals exhibiting particular traits in each generation, blending science and mathematics to arrive at the laws of inheritance.
Imp Concepts in Inheritance:
- Both father and mother contribute equally to the genetic material of the child
- Each trait can be influenced by both paternal and maternal DNA
- For each trait, there will be two versions in each child
Mendel's Pea Plant Experiments
Mendel used contrasting visible characters of garden peas:
| Trait | Contrasting Forms |
|---|---|
| Plant Height | Tall / Short |
| Seed Shape | Round / Wrinkled |
| Flower Color | Violet / White |
| Seed Color | Yellow / Green |
Monohybrid Cross: Tall × Short Plants
Imp Observations from F1 Generation:
- No "medium-height" plants – no halfway characteristics
- All F1 plants were tall
- Only one parental trait was seen, not a mixture
Imp Observations from F2 Generation:
- Not all F2 progeny were tall
- One quarter were short
- This indicates both tallness and shortness traits were inherited in F1, but only tallness was expressed
- Ratio: 1 TT : 2 Tt : 1 tt (genotypic ratio)
- Ratio: 3 Tall : 1 Short (phenotypic ratio)
Dominant and Recessive Traits
| Genotype | Phenotype (Appearance) | Explanation |
|---|---|---|
| TT | Tall | Both copies are for tallness |
| Tt | Tall | One copy of 'T' is enough to make the plant tall |
| tt | Short | Both copies must be 't' for short plant |
Imp Definitions:
- Dominant Trait (T): A trait that expresses itself even when present in single copy. Example: Tallness
- Recessive Trait (t): A trait that expresses itself only when both copies are present. Example: Shortness
Dihybrid Cross: Two Traits Together
What happens when pea plants showing two different characteristics are bred?
Cross: Tall plant with round seeds × Short plant with wrinkled seeds
F2 Results (Phenotypic Ratio):
| Phenotype | Ratio | Out of 16 |
|---|---|---|
| Round, Yellow | 9 | 9/16 |
| Round, Green | 3 | 3/16 |
| Wrinkled, Yellow | 3 | 3/16 |
| Wrinkled, Green | 1 | 1/16 |
Imp Ratio: 9:3:3:1
Imp Conclusion:
The appearance of new combinations (Round-Green and Wrinkled-Yellow) proves that:
- The tall/short trait and round/wrinkled trait are independently inherited
- Genes for different traits can recombine during reproduction
How do these Traits get Expressed?
The Mechanism:
- Cellular DNA is the information source for making proteins
- A section of DNA that provides information for one protein is called a gene
- Proteins control the characteristics we observe
- Plants have hormones that trigger growth
- Plant height depends on the amount of growth hormone
- An enzyme is important for making this hormone
- Efficient enzyme → More hormone → Tall plant
- Less efficient enzyme → Less hormone → Short plant
Conclusion: Genes control traits by controlling protein/enzyme production
Why Two Copies of Each Gene?
Imp Facts:
- Both parents contribute equally to the DNA of the progeny
- Each parent contributes one copy of each gene
- Each organism has two sets of all genes – one from each parent
- Germ cells (gametes) must have only one gene set
Chromosomes: The Carriers of Genes
How do germ cells make a single set of genes from two copies?
Imp Understanding:
- Each gene set exists as separate independent pieces called chromosomes
- Each cell has two copies of each chromosome – one maternal, one paternal
- During germ cell formation, each germ cell takes one chromosome from each pair
- These may be of either maternal or paternal origin
- When two germ cells combine, they restore the normal number of chromosomes
This mechanism ensures:
- Stability of DNA across generations
- Independent inheritance of different traits
- Genetic variation in offspring
Sex Determination
How is the sex of a newborn individual determined? Different species use different strategies:
| Method | Example | Mechanism |
|---|---|---|
| Environmental | Some reptiles | Temperature determines sex |
| Sex Change | Snails | Individuals can change sex |
| Genetic | Humans | Chromosomes determine sex |
Sex Chromosomes in Humans
Imp Facts:
- Humans have 23 pairs of chromosomes
- 22 pairs are autosomes (perfectly matched pairs)
- 1 pair consists of sex chromosomes (may not be perfectly matched)
- Females: XX (perfect pair of sex chromosomes)
- Males: XY (mismatched pair – X is normal, Y is shorter)
Imp Points About Sex Determination:
- All children inherit an X chromosome from their mother
- The father determines the sex of the child:
- Child inherits X from father → Girl (XX)
- Child inherits Y from father → Boy (XY)
- Probability: 50% boys and 50% girls
- The mother cannot be blamed for the sex of the child
Chapter Questions and Answers
Question 1
How do Mendel's experiments show that traits may be dominant or recessive?
Mendel's experiments demonstrated dominant and recessive traits through his monohybrid cross:
- F1 Generation observation: When he crossed tall pea plants (TT) with short pea plants (tt), ALL F1 offspring were tall, not medium-height
- This showed that tallness dominated over shortness
- F2 Generation observation: When F1 plants (Tt) were self-pollinated, the F2 generation had both tall and short plants in a 3:1 ratio
- The reappearance of short plants proved the shortness trait was present but hidden in F1
Conclusion:
- Dominant trait (T): Expressed even when present in single copy (Tt shows tall)
- Recessive trait (t): Expressed only when both copies are present (tt shows short)
Question 2
How do Mendel's experiments show that traits are inherited independently?
Mendel's dihybrid cross proved independent inheritance:
- Cross performed: Round, Yellow seeds (RRYY) × Wrinkled, Green seeds (rryy)
- F1 Result: All plants had Round, Yellow seeds (RrYy)
- F2 Result: Four types of offspring appeared:
- Round, Yellow (9)
- Round, Green (3) - NEW combination
- Wrinkled, Yellow (3) - NEW combination
- Wrinkled, Green (1)
Significance of new combinations: The appearance of Round-Green and Wrinkled-Yellow (which were not present in parents) proves that:
- Seed shape and seed color traits separated and recombined independently
- The two traits are controlled by genes on different chromosomes
- Inheritance of one trait does not affect inheritance of another
Question 3
A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?
No, this information is NOT enough to determine which trait is dominant.
Reason:
- The daughter has blood group O, which she must have inherited from both parents
- The mother (blood group O) can only give O allele
- The father (blood group A) must have given an O allele to the daughter
- This means the father's genotype could be:
- AO (heterozygous) - has both A and O alleles
- If A is dominant, genotype AO shows blood group A
- If O is dominant, genotype AO should show blood group O, but father shows A
What we can conclude:
- Blood group A appears to be dominant because the father with AO genotype shows blood group A phenotype
- However, to be completely certain, we would need to see results from multiple families or do more crosses
Actually, A IS dominant over O, but the single case doesn't provide conclusive proof - we need larger sample sizes for scientific certainty.
Question 4
How is the sex of the child determined in human beings?
In human beings, sex is determined genetically by sex chromosomes:
Chromosome composition:
- Humans have 23 pairs of chromosomes (46 total)
- 22 pairs are autosomes (same in males and females)
- 1 pair consists of sex chromosomes
| Parent | Sex Chromosomes | Gametes Produced |
|---|---|---|
| Mother (Female) | XX | All carry X chromosome |
| Father (Male) | XY | 50% carry X, 50% carry Y |
Mechanism:
- All children inherit one X chromosome from mother
- Children inherit either X or Y from father
- If child gets X from father → Girl (XX)
- If child gets Y from father → Boy (XY)
Conclusion: The father's chromosome determines the sex of the child, with a 50:50 probability of boy or girl.
Exercise Questions and Answers
Exercise 1
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as:
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Explanation:
- Given information:
- Tall parent with violet flowers crossed with short parent with white flowers
- All progeny have violet flowers (W is dominant)
- About half progeny are short (ratio is approximately 1:1 for height)
- Analysis for height:
- Short parent genotype: tt
- Half offspring are short means 1:1 ratio
- This happens when one parent is heterozygous (Tt)
- Tt × tt → 50% Tt (tall), 50% tt (short)
- Analysis for flower color:
- Short parent genotype: ww
- All offspring have violet flowers
- This means tall parent must be WW (homozygous)
- WW × ww → All Ww (violet)
Therefore, tall parent genotype = TtWW
Exercise 2
A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Reason:
- The observation only shows that light eye color runs in families - it's inherited
- This pattern would occur whether light eye color is dominant OR recessive
- If light is recessive:
- Parents with light eyes (ll) can only produce children with light eyes (ll)
- This fits the observation
- If light is dominant:
- Parents with light eyes (LL or Ll) would mostly produce children with light eyes
- This also fits the observation
What we need to determine dominance:
- We need to study families where parents have DIFFERENT eye colors
- If dark × light consistently produces dark children, dark is dominant
- If dark × light consistently produces light children, light is dominant
- We need F2 generation data to see phenotypic ratios
Note: In reality, dark eye color is dominant over light eye color in humans.
Exercise 3
Outline a project which aims to find the dominant coat colour in dogs.
Objective: To identify which coat color is dominant in dogs (e.g., black vs. brown)
Method:
- Select Purebred Parents:
- Choose purebred black dogs (BB)
- Choose purebred brown dogs (bb)
- Ensure they are from pure lines for at least 3 generations
- Cross Breeding (F1 Generation):
- Breed black dogs with brown dogs
- Observe the coat color of all F1 puppies
- All F1 puppies will show the dominant color
- If all puppies are black → Black is dominant
- If all puppies are brown → Brown is dominant
- F2 Generation (Confirmation):
- Breed F1 dogs with each other (Bb × Bb)
- Observe coat colors in F2 generation
- Expected ratio: 3 dominant color : 1 recessive color
- This 3:1 ratio confirms which trait is dominant
- Data Collection:
- Record number of puppies of each color in both generations
- Calculate percentages
- Use large sample sizes (minimum 50-100 puppies) for accuracy
- Test Cross (Additional confirmation):
- Breed F1 dogs (Bb) with recessive parent (bb)
- Expected ratio: 1 dominant : 1 recessive (50:50)
Expected Results:
| Cross | Result if Black is Dominant |
|---|---|
| BB × bb | All Black (Bb) |
| Bb × Bb | 3 Black : 1 Brown |
| Bb × bb | 1 Black : 1 Brown |
Precautions:
- Ensure purebred parents
- Maintain proper records
- Use large sample sizes
- Consider environmental factors that might affect coat color
Exercise 4
How is the equal genetic contribution of male and female parents ensured in the progeny?
Equal genetic contribution from both parents is ensured through the following mechanism:
1. Diploid Body Cells:
- All body cells are diploid (2n)
- They contain two sets of chromosomes - one from each parent
- In humans: 23 pairs = 46 chromosomes total
2. Meiosis - Formation of Gametes:
- Reproductive cells undergo meiosis (reduction division)
- During meiosis, chromosome number is reduced to half
- Gametes (sperm and egg) are haploid (n)
- Each gamete contains only one set of chromosomes
- In humans: gametes have 23 chromosomes (not 23 pairs)
3. Process:
- Male parent: Produces sperm cells (n) through meiosis
- Female parent: Produces egg cells (n) through meiosis
- Each gamete receives:
- One chromosome from each pair
- This can be either maternal or paternal origin
4. Fertilization:
- One sperm (n) fuses with one egg (n)
- The resulting zygote is diploid (2n)
- n (from father) + n (from mother) = 2n (in child)
Conclusion:
- The reduction of chromosome number during meiosis and restoration during fertilization ensures equal genetic contribution
- This maintains chromosome number constant across generations
- Child receives exactly 50% DNA from each parent
Imp Concepts Summary:
- Variations arise during reproduction and can be inherited
- Dominant traits express themselves even in single copy; recessive traits need both copies
- Independent inheritance allows traits to be inherited separately, creating new combinations
- Genes control traits by controlling protein production
- Chromosomes are carriers of genes; each cell has two sets
- Meiosis ensures gametes have half the chromosome number
- Sex determination in humans depends on whether the paternal chromosome is X (girls) or Y (boys)
- Both parents contribute equally to offspring DNA through fertilization